Library /sys$common/syshlp/ncphelp.hlb SET, CIRCUIT, MAXIMUM TRANSMITS number  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Applies to DDCMP CONTROL circuits.  It specifies the maximum
 number of data messages that can be transmitted at one time.
 Number must be a decimal integer in the range 1 to 255.  The
 default is 4.